rockfoot

2000 Mustang, V8, SOHC. Sometimes it starts and other times it will not even turn over. Sometimes the solenoid "clicks" and other times, it chatters. Sometimes it does nothing at all. Acting very much like dirty battery connections or loose leads. Then, all of a sudden, it works like it is supposed to for a few days and then starts acting up again. Have gone over the circuit to the starter many, many times with wrenches and meters. Is there a "fused wire" in this circuit that I am overlooking? It was doing this before a new engine was installed and we really had no problems after the engine was replaced. Here recently, when the weather turned, things started happening. The car is driven by a 17 year old that is pretty darn responsible and calls his Dad if there is the slightest problem. Short of replacing the starter and the battery (again) what could be the problem? Has anybody seen this before? Any help or direction will be appreciated. Thanks in advance.